CIC channels and transporters are found in all major groups of organisms and function in essential physiological processes. Mutations in human CIC genes give rise to a diverse array of diseases. Studies proposed in this application focus on defining the mechanism by which the Ste20 kinase GCK-3 regulates a C. elegans CIC anion channel, CLH-3b, that is activated by meiotic cell cycle progression and cell swelling. GCK-3 is an ortholog of mammalian PASK and OSR1. These three kinases have been shown to play critical roles in regulating the activity of cell volume sensitive Cl-transport pathways as well as transepithelial transport mechanisms that function in whole animal osmoregulation. Our recent studies have demonstrated that GCK-3 is essential for whole animal osmotic balance in C. elegans. Studies proposed in this application will use a combination of mass spectrometry, protein biochemistry, molecular biology, reverse genetics and patch clamp electrophysiology to 1) identify GCK-3 phosphorylation sites that regulate CLH-3b activity and 2) determine whether CLH-3b is phosphorylated directly by GCK-3 and identify other signaling pathway components. Results of studies proposed in this application will continue to broaden our understanding of CIC anion channel biology and will provide a deeper molecular understanding of CIC regulation and of how cells sense size changes and regulate salt and water transport. Detailed understanding of CIC channel function/regulation and GCK-3/PASK signaling is essential for understanding epithelial transport disorders such as cystic fibrosis, secretory diarrhea, endolymphatic deafness, polycystic kidney disease, and diseases that disrupt kidney salt and water transport processes. Lay description: CIC anion channels are proteins that transport Cl- across cell membranes and play essential roles in numerous physiological processes. Mutations in genes that encode human CIC channels give rise to kidney, bone, muscle and neurological diseases. My proposed studies will define how a Ste20 kinase regulates a CIC channel. Ste20 kinases play critical roles in regulating cell cycle events, cellular stress responses and kidney salt and water transport processes. Results of studies outlined in this application will provide new insight into epithelial transport disorders such as cystic fibrosis, secretory diarrhea, endolymphatic deafness, polycystic kidney disease, and diseases that disrupt kidney salt and water transport processes.